Publications

4851

The bois noir (BN) disease of grapevines is widespread in German winegrowing regions. It is associated with ‘Candidatus Phytoplasma solani’, which affects not only grapevines but also other wild and cultivated plants. This pathogen has a complex epidemiology including different insect vectors and various host plants. A study was carried out to investigate the genetic variability of ‘Ca. P. solani’ in different winegrowing regions in Germany. Between 2017 and 2023, samples of grapevine, stinging nettle, bindweed, and other herbaceous plants as well as specimens of different planthopper species colonizing viticultural habitats were analyzed for infection with ‘Ca. P. solani’. All positive tested samples were further characterized by multilocus sequence typing (MLST) based on the genes tuf, stamp, secY, and vmp1. The genetic variability was assessed by RFLP analyses of the tuf and vmp1 PCR products, coupled with sequencing of the stamp and secY amplification products. A total of 1274 grapevines, 35 bindweed, and 18 stinging nettle samples were infected with ‘Ca. P. solani’ but also five samples of other weed species. Among the known and putative insect vectors, specimens of Hyalesthes obsoletus, Reptalus spp., and Dictyophara europaea harbored the phytoplasma. In both plants and insects, two genotype combinations were predominantly associated with the classical bindweed and stinging nettle cycle, respectively. The MLST analysis revealed considerable differences between German isolates and data reported from other European regions and new genotype combinations were identified, indicating new host plant–vector associations.

‘Candidatus Liberibacter asiaticus’ (CLas) is a devastating pathogen of citrus associated with Huanglongbing (HLB, citrus greening disease). HLB is economically significant in Asia and has destroyed millions of citrus trees worldwide during the last century. Since 2007, when the first molecular evidence for the presence of CLas in North-West Frontier Province of Pakistan (Khyber Pakhtunkhwa) were presented, there have been a limited number of studies reporting the pathogen in different citrus growing districts of the Punjab Province, one of the major citrus producing areas of the country. In this study, a small-scale survey was conducted in citrus groves exhibiting HLB-like symptoms in different districts of Punjab Province. The aim was to obtain current information on the incidence of CLas in the area and complement the previous studies. Conventional and real time quantitative polymerase chain reaction (PCR and qPCR) assays were used for the detection of CLas in the 94 samples tested, followed by sequencing of the PCR amplicons. Overall CLas was detected in 43% (40/94) of the tested samples from Kinnow mandarin (n= 22), sweet orange (n= 12), grapefruit (n= 3) and Feutrell’s early mandarin (n= 3). In agreement with previous reports, this survey confirmed the presence of HLB in the Punjab Province of Pakistan and provided updated molecular evidence for the presence of CLas and disease incidence for four citrus cultivars from five citrus growing districts of the province.

ABSTRACT Citrus huanglongbing (HLB), caused by the phloem‐limited bacterium ‘ Candidatus Liberibacter asiaticus’ ( C Las), is one of the most destructive diseases in global citrus production. Here, we report the functional characterization of a core C Las effector, CLIBASIA_00525 named SDE525, that manipulates host immunity by targeting the citrus nascent polypeptide‐associated complex alpha subunit (CsNACα). We confirmed that SDE525 is a secreted protein that localizes to the nucleus, cytoplasm, and plasma membrane. Notably, the effect of SDE525 exhibits host‐specific context dependence: while its transient expression in the non‐host Nicotiana benthamiana unexpectedly triggered immune responses, in its native host, sweet orange ( Citrus sinensis ), SDE525 interacted with CsNACα to modulate defence, as demonstrated by multiple independent assays. Integrated transcriptomic and metabolomic analyses revealed that this interaction reprograms the host's hormonal landscape, suppressing the jasmonic acid (JA) signalling pathway while simultaneously activating the salicylic acid (SA) pathway. Importantly, under transient overexpression conditions in citrus, this immune reprogramming enhanced resistance against the bacterial canker pathogen Xanthomonas citri subsp. citri —a phenomenon distinct from the susceptibility observed during authentic C Las infection. Our findings uncover a sophisticated mechanism by which a C Las effector rewires host defence signalling, providing fundamental insights into its pathogenesis with potential long‐term implications for future HLB management.

Citrus huanglongbing (HLB), associated with the bacterium ‘Candidatus Liberibacter asiaticus’ and spread by the Asian citrus psyllid (Diaphorina citri; ACP), poses a significant threat to California’s citrus industry. First identified in Los Angeles in 2012, HLB has since spread through residential areas across Southern California. A risk-based survey (RBS) model has been developed to improve HLB surveillance and intervention. Within this framework, model components change as HLB dynamics shift, requiring regular updates to maintain data accuracy and model reliability. Disease spread is influenced by natural factors, such as ACP establishment and confirmed HLB locations, as well as human-mediated factors like global mobility (travel introduction from HLB-infected countries), transportation of citrus materials, nurseries, packinghouses, farmers’ markets, and proximity to private or otherwise inaccessible lands. Human-mediated risk factors account for approximately 26.3% (18.4 to 38.4%) of HLB incidence across different years, and natural causes predominantly explain the remaining 73.7% (61.6 to 81.7%). Notably, global mobility was crucial for early HLB detection in new areas, whereas ACP density strongly correlated with disease spread once established. A retrospective analysis from 2015 to 2022 evaluated the RBS model’s performance, showing a predictive power of 88 to 97%, which confirms its validity for developing targeted interventions and early detection strategies in California.

ABSTRACT Insect‐vectored pathogens pose a significant threat to global agriculture. The colonization efficiency of pathogens in vectors plays a central role in these pathosystems, yet studies of the factors that affect this aspect are limited. This study investigates the genetic and microbial symbiont factors influencing the susceptibility of Diaphorina citri to Candidatus Liberibacter asiaticus ( C Las), the pathogen causing citrus greening disease (huanglongbing). Through a microbiome Genome Wide Association Study (mGWAS) based on 16S amplicon sequencing and genomic resequencing of 120 D. citri individuals from six populations, we identified 79 SNPs significantly associated with the relative abundance of C Las within insects. Additionally, some of these SNPs were also associated with the relative abundance of Candidatus Profftella armature, a key endosymbiont of D. citri . SNPs in the regulatory region of gene Dcitr04g11610.1 led to its overexpression in C Las‐susceptible D. citri , and C Las infection further elevated its expression. Conversely, RNAi knockdown of Dcitr04g11610.1 reduced C Las infection rates and abundance, accompanied by increased abundance of Profftella . Phylogenetic analysis revealed Dcitr04g11610.1 ’s high homology to Major Facilitator Superfamily‐type transporter SLC18B1 proteins, suggesting a role in C Las polyamine utilization. These findings highlight the importance and potential interplay of insect genetics and symbiotic microbiota in insect‐vectored plant pathogen systems.